A lithium ion battery has been used mainly for a mobile purpose as a lightweight electricity storage device having a high energy density. In recent years, the lithium ion battery is used for a stationary power supply, an electric vehicle, a hybrid vehicle, a hybrid railway vehicle, and other such purpose that requires a large amount of energy.
A lithium phosphate compound having an olivine-type crystal structure, for example, LiFePO4 or LiMnPO4, is high in theoretical capacity, low in cost, and excellent in thermal stability at a time of charging, and is therefore attracting attention as a promising positive electrode active material.
However, a lithium ion battery using an olivine-type phosphate compound, for example, LiFePO4 or LiMnPO4, for a positive electrode has a stable voltage over a wide range, and exhibits substantially no voltage fluctuations due to a change in SOC. This raises a problem in that it is difficult to detect an SOC from the voltage.
A lithium-containing titanium oxide or a carbon material, for example, graphite-based carbon or low-crystalline graphitizable carbon (soft carbon) or non-graphitizable carbon (hard carbon) subjected to low-temperature baking, is used as a negative electrode active material. In particular, graphite-based carbon is often used as the negative electrode active material.
A lithium ion battery using low-crystalline soft carbon or hard carbon exhibits a large voltage change due to charging/discharging. However, a lithium ion battery using a graphite-based material or a lithium-containing titanium oxide exhibits a small voltage change, and hence it is difficult to detect the SOC from the voltage.
In particular, in a case of using a lithium ion battery (hereinafter referred to as “iron-phosphate-based lithium ion battery”) using an olivine-type phosphate compound as a positive electrode active material and graphite-based carbon as a negative electrode active material, a battery voltage is in a constant state over a wide range, which raises a problem in that it is difficult to detect the SOC.
In general, the following methods are known as a method of estimating the SOC of a battery:                an estimation method of creating OCV-SOC characteristics by measuring a relationship between an open circuit voltage (OCV) and an SOC of a battery to be used, and determining the SOC with reference to the OCV-SOC characteristics;        an estimation method of integrating a discharging current starting from a full charge state; and        an estimation method of integrating a charging current starting from full discharge.        
Specific examples of estimating the SOC of the battery include the following related art (see, for example, Patent Literature 1). In Patent Literature 1, an OCV voltage is measured by shifting the SOC of the iron-phosphate-based lithium ion battery through charging or discharging to a high-SOC area exhibiting a large voltage change around full charge or an area exhibiting a large voltage change near a full discharge state at a last stage of the discharging, and the SOC is detected with reference to the OCV-SOC characteristics.
Meanwhile, Patent Literature 1 employs a method of estimating the SOC through current integration in an area corresponding to a large part of the OCV-SOC characteristics and exhibiting small voltage fluctuations.